This study is directed toward achieving the long-term goal of describing how early developmental decisions are made at the cellular level in mammalian embryos, and how these decisions being on the events that occur during the peri-implantation stages. The realization of this goal will be helpful in understanding the bases of successful and unsuccessful implantation, as well as the causes of developmental anomalies that occur during morphogenesis in the mammalian early embryo. Specifically, this proposal will investigate the positional changes that accompany the allocation of cells to different fates in the trophectoderm of the 3.5- to 6.0 day-old mouse embryo. The trophectoderm cells of the 3.5-day-old mouse embryo are the known progenitors of a diverse set of disparately situated extra- embryonic tissues that perform vital but separate functions during implantation. The allocation of these contiguous epithelial cells to their various fates depends on their position in the mouse embryo Cell position, in turn, is affected by such factors as mitotic rates, cell shape changes and cell translocation within and into the trophectoderm itself. This proposal will evaluate the extent to which each of these factors influences cell fate in the trophectoderm of peri-implantation mouse embryos. The experimental approach centers around the individual labeling of positionally identified trophectoderm cells by intracellular iontophoretic microinjection of such marker substances as horseradish peroxidase, rhodamine dextran and novel formulations of standard histological dyes. The position and fate of histochemically marked cells and their descendants will be determined by in toto visual evaluation under magnification of pre-implantation embryos incubated in vitro. To analyze cell position and fate in post-implantation embryos, microinjected 3.5- day-old embryos bearing marked cells will be transferred to pseudopregnant foster mothers and the resulting conceptuses will be retrieved about two days later, serially sectioned and immunohistochemically processed to detect marked cells and their descendants. The data obtained will form the basis for a model explaining how cell position influences cell fate in the trophectoderm of the developing mouse embryo.